CONSLQUENCl:S OF CHANGl: 



of these plants is not disturbed too much to prevent them providing 

 some of our best garden varieties. Thus in this species the chromo- 

 somes are less sharply differentiated than usual. Or, to put it the 

 other way round, each chromosome is balanced in much the same 

 way as the chromosome set in the aggregate is balanced. The fertility 

 of the triploid and the normality of the trisomic follow from this 

 unusual condition. 



Structural Hyhridity 



The product of structural change in the chromosomes, like the 

 product of numerical change, reveals its peculiar properties at 

 meiosis. We then see that it is a structural hybrid, an individual 



UnchanjeS 

 Chromosomes 



Clirs. af mitosis after Pairing of two inferclian^eS 



Intercnan^chif DKaUage Vtwo unchanged Chrs. 

 VRcunlon of Chromatids aff^e ?<ichytene slaje of 

 rieio5Js in an interchange 



t A At 



I c c I 



Non- Disjunctional 



-giving sterility 



hybrid.^ 



^ 4, 



Disjunctional 



-giving Fertility 



The same after Cro5sing Alternative Co'orientations of the Riag of four 

 OverV Chiosmo formoffon on ti\ct^etaphaic \ylafe. in sibe vie« 



Fig. 29. — The consequences of interchange between two non-homologous chromo- 

 somes AB and CD at the succeeding mitosis and at meiosis where the zygote is in 

 consequence heterozygous and forms a ring of four chromosomes which, in a pro- 

 portion of cells, by failure of disjunction of alternate chromosomes, gives sterile 

 haploid products. 



128 



